Micro-gasifiers were used extensively in Europe during World War II to power internal combustion engine vehicles while conventional gasoline was very difficult to obtain. The basic operating principles of such devices will now be described. Initially, dry biomass is combusted in an enclosed container under a mild vacuum as generated by the intake manifold of a reciprocating piston engine. Air inlets and gasifier output stream connections are arranged so that the biomass is only partially combusted, resulting in an exhaust stream which contains carbon monoxide and may additionally contain hydrogen and hydrocarbon gasses. This gas can be further combusted in an internal combustion engine to produce shaft power.
FIG. 1 (prior art) illustrates a typical layout of such devices including gasifier, filter, startup and turn off flare, air mixing valves and an internal combustion engine. The normal startup procedure is to light the gasifier and bring it up to a temperature which produces a sufficient amount of combustible gasses to at least idle the attached internal combustion engine. This is typically done by initially routing the output of the gasifier to a flare device, which protects operating personnel from the highly toxic carbon monoxide gas generated by the gasifier. Various schemes for initiating airflow through the gasifier are used such as incorporating an aspirating pump in the flare device or an inline blower in the flare device, thus creating a partial vacuum in the gasifier. Alternatively, an input blower can be utilized on the gasifier to force air through the system. However, this is generally regarded as less desirable because positive pressure in the gasifier device can result in highly toxic carbon monoxide gas leakage from various system components such as monitoring ports, biomass feed and ash ports, and various system interconnections.
Once airflow is set up through the gasifier, its combustion zone can be lit by any of the various techniques used to start a wood fire, with initiation by a propane torch device being one of the most common techniques in current use. Dependent upon the size of the gasifier, and the oxidation state and moisture content of the biomass fuel located in the gasifier, startup will typically take three to 30 minutes. In order to maintain area safety, the flare should be equipped with an igniter which burns escaping carbon monoxide gas. Once a sufficient quantity of combustible gases are present in the gasifier output stream, the flare valve is closed, the genset gasifier valve is opened and the attached engine is cranked with dynamic adjustment to the air inlet valve in order to provide a combustible mix suitable for firing the engine cylinder(s). In normal operation the engine's displacement revolution rate (RPM) and load demands provide a degree of regulation the input airstream to the gasifier and thus the rate at which biomass is consumed in its internal partial combustion process. This partial combustion process typically consumes all of the oxygen in that input stream so the air inlet valve on the engine is adjusted to provide enough oxygen for a suitable air fuel ratio for the desired output power.
On turn off the engine's ignition system and/or its air supply valves are turned off, which stops the engine and stops flowing air through the gasifier. The gasifier core temperature may be well above 1,000 degrees centigrade and the system may be equipped with multiple layers of insulation so that it will typically take several hours for the gasifier to cool down to room temperature. Restart delay is typically directly proportional to the amount of time the gasifier has been off, with shut down durations of a few minutes resulting in nearly instantaneous restart due to the residual combustible gases retained by the system and the high combustion zone core temperature.
Internal combustion engines typically produce peak operating efficiency at a specific design point. Such a design point is dominated by frictional and accessory losses on the low side and non-optimal combustion dynamics on the high end particularly if the engine designers have pushed the peak output rating of the engine past the optimal combustion operating region. Likewise, the gasifier is limited by heat losses on the low end which will limit the internal core temperature and, thus, the gasification rate and quality. At the high end, gasifier system constraints like biomass mass flow, air mass flow, air jetting geometry and hot zone geometry limit the gasifier's performance. Consequently, the combination of the gasifier and internal combustion engine will typically result in a fairly narrow power range for peak operating efficiency.